Magnetic characterization of reduction in Mount Fuji basaltic tree-mold

A Mount Fuji basaltic tree-mold presents the mineralogical record associate d with a large oxygen fugacity gradient during the cooling of the basalt. M agnetic exchange anisotropy is postulated to be characteristic of cation ex cess in some spinel titanomagnetites. The exchange anisotropy is characteri zed by loop shifting along the field and magnetization axes when cooled in a magnetic field, large coercivity increase, and magnetic viscosity which h as discrete activation fields and logarithmic to linear decay characteristi cs. The cryogenic loop characteristics attributable to exchange anisotropy are not found in the tree-mold where iron and ilmenite are the main phases. The coupling has been created in the laboratory with ordinary basalt artif icially reduced in Hydrogen gas. The coupling is not found in spinel oxidat ion non-stoichiometry. These results imply that we are able to identify the presence of spinels that have equilibrated in the low oxygen fugacity rang e of the spinel stability field where cation excess is characteristic.